1. Field of the Invention
The present invention relates to mono- and diarylamide perfluoropolyether compounds and to compositions thereof with perfluoropolyether fluids, said compositions having improved resistance to degradation caused at high temperatures by the action of metals and oxygen and by metal oxides. The compositions are prepared by adding arylamide derivatives of mono- and dicarboxylic perfluoropolyether acids to the perfluoropolyether fluid.
2. Description of the Prior Art
The continuing development of the technology of processing machinery and of transportation, especially in the aeronautical field, requires metal materials capable of withstanding stresses at higher and higher temperatures. In particular applications, these metal materials are in contact with non-metallic materials, such as antifriction agents, lubricants and heat exchange means all of which must be capable of resisting vigorous operating conditions. In particular, lubricating fluids having greater thermal stability and resistance to oxidation than those of the conventional products now available are required.
The requirement for lubricating and heat exchange fluids capable of withstanding, for long periods and without excessive alteration, high temperatures and high shearing stresses, has led to the synthesis and formulation of new fluids.
For this purpose, perfluorinated compounds are particularly suitable because of their high resistance to high temperatures. The perfluorinated polyethers constitute one of the most important classes of high viscosity perfluorinated fluids, since they combine the good characteristics of thermal stability and oxidation resistance with viscosity indices and low temperature characteristics comparable to or better than those of mineral oils.
The polyperfluoroether fluids, mixtures thereof and processes for their preparation have heretofore been described, for instance, in Italian Pat. No. 793,154; Canadian Pat. No. 810,750; and U.S. Pat. No. 3,214,478.
They consist of linear molecules containing, in their chain, repeating perfluoroalkylene units, which may be the same or different from one another and which contain from 1 to 3 carbon atoms, mutually linked through an ether linkage; generally, the terminal groups of the chains are perfluoroalkyl radicals.
The polyperfluoroether fluids have the following general formula: EQU X--O--(C.sub.3 F.sub.6 O).sub.P --(CF.sub.2 O).sub.q --(C.sub.2 F.sub.4 O).sub.r --Y (I)
wherein --C.sub.3 F.sub.6 -- and C.sub.2 F.sub.4 -- are perfluoroalkylene groups derived, respectively, from the opening of the double bonds of hexafluoropropylene and tetrafluoroethylene, respectively, and when different perfluoroalkylenoxy units are simultaneously present, they are randomly distributed along the chain; p, q and r are average composition indices, the sum of which is between 6 and 200; each of p, q and r can be zero but not simultaneously or a number ranging from 1 to 100; X and Y are identical or different terminal groups and generally are perfluoroalkyl radicals containing from 1 to 3 carbon atoms. These polyperfluoroethers containing from 6 to 200 perfluoroalkylenoxy repeating units are endowed with fluidity characteristics at room temperature which are typical of either fluid or lubricating greases or hydrodynamic or heat exchange fluids.
Some of these fluids are stable for long periods of time at temperatures of 420.degree.-450.degree. C. However, it has been found that at temperatures around 250.degree. C., i.e. at temperatures considerably lower than that of the thermal stability of the bonds present in the chain, these fluids, when put into contact with certain metal materials in common use and in the presence of air, undergo remarkable degradation such as a decrease in viscosity and a loss of weight of the material in the form of poisonous and corrosive gases.
Moreover, in the thus degraded fluid, there appear acid type functions which may have corrosive effects on the metals. Particularly severe degradative effects have been observed in the presence of titanium, aluminum, alloys thereof and certain steels.
Similar degradation phenomena occur to a greater extent when the perfluoropolyether fluids are heated at temperatures higher than 250.degree. C. in the presence of metal oxides. In addition, it has been noted that, contrary to what occurs in the presence of metals, degradation also occurs in the presence of oxides in a non-oxidizing atmosphere such as, for instance, under vacuum or in a N.sub.2 or CO.sub.2 atmosphere.
In view of this degradation, it is necessary to incorporate into the fluid small amounts of materials having functional groups which exert a stabilizing action with respect to the above mentioned degrading agents. For this purpose, it is known that additives, i.e., chemical compounds which exert specific and characteristic actions directed toward improving certain properties of a fluid, are largely used in the field of hydrocarbon based normal fluids and lubricating oils.
On the other hand, in the present case, for various reasons, it is not always convenient to employ as stabilizing additives those compounds normally used for the stabilization of hydrocarbon fluids. In fact, in order to be useful as a stabilizer for perfluoropolyether fluids, the stabilizing compound must be compatible with and sufficiently soluble or miscible in said fluids at room temperature, and it is known that the perfluorinated liquids have very poor compatibility and miscibility with respect to non-completely fluorinated compounds. Furthermore, the stabilizing compound must itself have a very high thermal resistance and a minimun volatility because of the high temperatures at which it is used; and it must not significantly alter, by its presence, the basic properties of the fluid, such as for instance, the viscosity, the lubricating power and the resistance to chemical agents.
Lastly, during its action, the stabilizer must not give rise to any significant carbon formation or formation of fluid-insoluble by-products.